Radio Shack Rambles

Having recently had another IC-735 on the bench for repair, this time with a faulty main encoder, I decided to make a modern replacement for the small pcb that sits on top of the encoder metal enclosure. The encoder has a rotating disc with graticules that pass between a couple of IR LEDs and corresponding IR detectors. There is a small pcb that sits on top of the encoder that then generates two streams of pulses (one set inverted), as shown in the schematic below. On my unit, both of the receivers had failed. A good search failed to come up with anything suitable as a replacement, mainly because the originals have a very small form factor. I did find some photo transistors of a suitable size, but the output from these had rather long rise and fall times and did not work well when tried out with the radio. So I came up with this design: (Note that the 4457P's are actually photo transistors, not LEDs)   This uses two pairs of IR LEDs and photo transistors feeding a 74HC14 Schmitt tr...

The TDS1002 is a 60Mhz, 1G/S 'scope, but the hardware is capable of running up to 100MHz, when it is then badged as a TDS1012. These notes show how I upgraded my 1002 to a 1012. The procedure to upgrade has two main steps, first update the 'scope model number via the RS232 interface, then run it through the factory calibration procedure. It will then operate as a TDS1012. My understanding (but I have no direct experience of other models) is that the same procedure is used for the TDS1000 and TDS2000 series of 'scopes. The first step is relatively easy, the second not so. There are a total of 70 steps to perform, first by inputting various DC voltages, then by applying various RF inputs. It took me several attempts to finally get this all to happen without the 'scope complaining and failing a step, and here are the main take aways: When any one step fails, you have to go right back to the begining and start again. The input impedance of the 'scope varies during some ...

This nice little rig from Alinco is on the bench for repair. Initial checkes showed it pretty much dead on arrival, except for the LCD backlight on one side. It was understood that it may have suffered lightning damage. First checks with the covers off showed IC206 (7808 voltage regulator) to be open circuit. I have to say that this rig is a nice piece of kit to work on, with easy dissasembly. So removing this pcb was pretty straight forward and a new 7808 was fitted. But there was still no life from the set.  Next step was to remove the front panel assembly and check the voltage rails for the logic system. Here R446 was found to be high in value and when applying power direct to the 7805 voltage regulator, it was running hot. So I replaced the resistor and the voltage regulator. Now the logic rails are correct and the processor runs at switch-on, with the display text appearing -  but the info on the display flashes. There is also no audio. The UHF board with the regulator on...

 The Marconi 2955 Receiver & Transmitter Test Set This is my story repairing one of these endlessly useful pieces of test equipment from the 1980's. Like the one in the photo, mine is an early example, with no letter suffix in the model number.  And not as clean and tidy looking either - in fact it was pretty grubby outside and very grubby inside. Some of the buttons have been used so often, that the lettering has rubbed off. So a well used example for sure. At switch on, the cooling fan would run, but nothing else - cold and dark as they say. These units have a 10MHz reference oscillator, oven controlled, and, as well as providing an RF reference, is used to derive the clock signal for the microprocessor. For some weird reason it is fitted to the EHT Supply board for the CRT - but anyhow, checking the output signal on this board showed no activity. The Reference Oscillator Removing the EHT Board is not too difficult and then the oscillator module, housed in a small greay ...

Retro Chip Tester - Firmware Upgrade with Raspberry Pi https://8bit-museum.de/sonstiges/hardware-projekte/hardware-projekte-chip-tester-english/ As a certified addict of vintage test equipment, I have been a big fan of Stephan's chip tester for some time now. I use it a lot to check TTL logic chips as good or bad. Also SRAM from the classic Tektronix TDS 'scope series. It is quick and easy to use.  I have had mine for a while now, and have been aware that a number of new firmware releases exist since mine arrived. From a quick read of the instructions, however, it was clear that a bit of learning and some kit would be needed, so never got around to doing it. Until now. Over the years I have developed a keen dislike for MS Windows, so am predominantly a Mac and Linux user. This does have a downside, in that a lot of tech nerdy stuff revolves around Windows. Another reason to delay launching in to the firmware upgrade. However, I have now successfully done it using a Raspberry Pi...

 This unit was an eBay purchase with no known history. These handy boxes have two banks of switched attenuators, one going from 0 to 22dB in 1dB steps (on the right), and the other from 0 to 120dB in 20dB steps. Nice. On this one, the 1dB stepped attenuator checked out fine, but on the other, the first 20dB attenuator had a problem; input impedance was 150 Ohm. Measured on a NanoVNA this gave a poor input match (no surprise) and an insertion loss of 16dB, with the next few attenuators correctly showing 20dB steps. Removing this bank of attenuators from the box was fairly straightforward, that is, once I had tracked down a 1/16th hex key to loosen the grub screws on the front knob (everything on this unit has an imperial thread). It is then a simple matter of removing a bunch of screws to release the bottom panel. Immediately obvious was the burnt out first resistor. The 20dB attenuators are made up as a pi network with 61 Ω   resistors on the two legs.  I happened t...